This invention relates generally to frequency band dividing filters using delay-line filters, and more particularly to a filter for the operation of frequency band dividing an input signal into band divided signals and then combining these signals into a combined signal, which filter includes delay-line filters having flat delay characteristics over the entire frequency range, thereby showing good cut-off characteristics, and operates to divide the input signal into a plurality of frequency band signals in a manner such that both the frequency amplitude characteristic and the frequency delay characteristic of the resulting combined signal will be flat.
In general, in a system such as an audio speaker system, there is a system in which speakers respectively for low, medium, and high frequency bands (hereinafter referred to as low, medium, and high bands), for example, are used, and an audio signal is divided into low, medium, and high bands, signals of the bands thus divided being supplied to the speakers for the respectively corresponding bands. For this band division, a frequency band dividing filter is used. In this case, the listener hears sounds resulting from the acoustical combination in the sound field of sounds reproduced by the speakers for the three bands. Furthermore, a system for dividing in this manner a signal into frequency bands, transmitting the signals thus divided through transmission systems respectively for the bands, and thereafter again adding and combining the signals thus transmitted thereby to obtain transmitted signals is being used also in systems such as a limiter or a noise reduction system.
In such cases, it is desirable that the characteristics of the signal obtained after being frequency band divided, transmitted, and then added and combined are equal to the characteristics of the signal prior to the frequency band division. Accordingly, a band dividing filter for accomplishing the above described frequency band division is required to have a sharp or steep cut-off characteristic and a frequency amplitude characteristic (hereinafter referred to as "amplitude characteristic") and a frequency delay characteristic (hereinafter referred to as "delay characteristic") such that the amplitude characteristic and the delay characteristic of the signal after adding and combining will respectively become flat.
However, in a typical frequency band dividing filter known heretofore, if any of the above mentioned cut-off characteristic, amplitude characteristic, and delay characteristic is as desired, the others are poor, and there have been none in which all of these characteristics are fully satisfactory.
Accordingly, the present applicant has previously proposed, in the specification of U.S. Pat. application, Ser. No. 888,561, filed Mar. 20, 1978, now U.S. Pat. No. 4,137,510 now allowed, a frequency band dividing filter capable of overcoming the difficulties accompanying the known frequency band dividing filters described above.
This proposed frequency band dividing filter comprises at least two unit circuits connected in cascade connection of at least two stages, and at least one supplemental delay circuit. Each of the unit circuits comprises a low-pass filter having a specific filtering frequency band with a steep cut-off characteristic, a delay circuit connected in parallel with the low-pass filter and having a frequency phase characteristic substantially equal to the frequency phase characteristic of said low-pass filter in the pass band thereof, and a subtraction circuit connected to the output sides of the low-pass filter and the delay circuit thereby to carry out subtraction of the outputs of the low-pass filter and the delay circuit. The supplemental delay circuit is connected in parallel with the second stage unit circuit, which is connected to the first stage unit circuit in cascade connection, and has the same characteristic as the characteristic of the delay circuit of the second stage unit circuit.
In accordance with this proposed frequency band dividing filter, the input signal can be so frequency band divided that, when the frequency band divided signals are combined, the frequency amplitude characteristic and the frequency delay characteristic of the combined signal are both flat. However, while this proposed frequency band dividing filter has an excellent cut-off characteristic particularly with respect to high frequencies, it entails a problem in that its delay characteristic is not constant throughout the full frequency band since the phase characteristic of the low-pass filter differs from that of the delay circuit.